Method, apparatus, header, and composition for ground heat exchange

ABSTRACT

A subterranean ground heat exchange system, a method of installation, and a grout composition therefor. The grout composition is a pumpable slurry formed of from about 70 to about 85 parts by weight natural flake graphite and from about 30 to about 15 parts by weight bentonite. The solids content of the pumpable grout slurry is preferably at least 35% by weight and is more preferably at least 40% by weight. The ground exchange apparatus preferably utilizes an improved supply and return header comprised of supply and return ports which are provided through the vertically extending outer wall of a header housing. The header also includes an interior supply conduit which extends from the supply port into the interior of the header housing and includes a bend positioned in the interior of the housing for directing the heat transfer fluid downwardly.

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 12/259,878, filed Oct. 28, 2008, which claims thebenefit of now expired U.S. Provisional Patent Application Ser. No.61/042,912 filed on Apr 7, 2008. This application also claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/158,556 filedMar. 9, 2009, and incorporates said Provisional Application Ser. No.61/158,556 by reference into this document as if fully set out at thispoint.

FIELD OF THE INVENTION

This invention relates to the exchange of heat with the earth forheating, cooling and thermal energy storage applications. In oneparticular aspect, but not by way of limitation, the invention relatesto cost effective materials, devices, systems, compositions, and methodswhich lower the thermal resistance between the working fluid (i.e., theheat exchange fluid) of a closed loop ground heat exchanger and theearth and to working fluid supply and return headers for concentricsubterranean ground heat exchange systems.

BACKGROUND OF THE INVENTION

Ground heat exchangers provide more efficient heating and cooling ofbuilding spaces by exchanging heat based on the average yearlytemperature of the soil and on a higher thermal conductivity of soil ascompared to air. The relatively constant temperature of the earthprovides a more favorable temperature gradient for heat transfer forboth heating and cooling than conventional atmospheric air sourcesystems because the atmospheric air experiences an average dailytemperature swing of 20° F. and an average seasonal temperature swing of80° F. on the North American continent. Heat is rejected to the earth bythe heat exchange fluid when in the cooling mode and absorbed from theearth by the heat exchange fluid when in the heating mode. Ground heatexchange is synonymous with the terms geothermal, shallow geothermal,ground source and geoexchange when used in the context of subterraneanheat exchange with the earth at the earth's ambient temperature.

Ground heat exchange systems can provide direct cooling or heating to abuilding space so long as an appropriate temperature gradient existsbetween the working fluid used in the system (e.g., water) and the earthand the ground loop is large enough to handle the heat load. The groundloop comprises the buried piping for the ground heat exchanger and fordistribution of the working fluid. For most urban applications, a heatpump is also typically installed in the system to increase the thermalgradient to provide “on demand” efficient heating and cooling to abuilding space. The heat pump greatly increases the load capacity of theground loop so that residential and business customers can afford thecost of installing the ground loop for their homes or businesses.

Various methods have been developed to exchange heat with the earth.Both vertical and horizontal pipe installations have been used to makesubterranean ground loops. Experience has shown that horizontal loopsare inefficient ground loops because the shallow depth of burial causesthe ambient soil temperature to track the surface ground temperature.Horizontal loops buried below the frost line are, however, excellent formelting snow on pedestrian pathways and removing ice from bridges.Vertical ground loops can be open or closed. An open loop is where atleast two wells are completed in a high productivity aquifer and wateris circulated from one well to another. This method can be no longerused in urban areas due to drinking water safety standards enacted toprevent aquifer contamination.

The vertical closed ground loop heat exchanger uses piping inserted in adrilled hole in the ground. The configuration of the pipe loop is eitherside-by-side (U-tube) or concentric. The pipe loop can be made of metalor plastic. Initially, metal pipe loops were used in both concentric andU-tube installations to save capital cost, but experience has shown thatmetal pipe loop installations eventually fail due to anodic corrosionfrom conducting telluric or man-made electrical currents from oneformation layer to another. Experience has also shown that plasticground loop installations can last indefinitely, but the local groundtemperature will heat up or cool down if the seasonal load is notbalanced.

Currently, the most common type of vertical closed loop ground heatexchanger is a U-tube installation, which consists of inserting twolengths of high density polyethylene (HDPE) pipe, with a U-bend joint onthe bottom, into a 4 to 6 inch diameter borehole. The borehole depthtypically ranges from 150 to 400 feet deep into the earth. To preventaquifer contamination, the bore hole is backfilled with impermeablegrout formed of a high solids bentonite slurry or neat cement. The groutbackfill keeps the piping in thermal contact with the wall of theborehole and provides a permeability barrier to reduce the verticalmovement of ground water from one aquifer to another or to preventsurface water contamination of an aquifer.

The vertical, closed-loop, ground heat exchanger typically uses water ora water antifreeze mixture as a working thermal fluid. Refrigerants suchas Freon® typically are not used due to expense and possible aquifercontamination. The water based fluid is circulated through the closedpiping system, which consists of a distribution system to the verticalwellbores. The wellbore loop provides a downward path and an upward paththat is arranged in either a U-tube or concentric pipe configuration.The U-tube configuration is about 30-60% as efficient as the concentricpipe configuration because, in the U-tube configuration, the returningfluid will reabsorb about 50% of the heat transfer to the ground on theway back up.

The concentric pipe configuration comprises a smaller diameter pipearranged concentrically within a larger diameter outer pipe (i.e., the“casing”). The inside surface of the smaller diameter pipe provides acenter flow channel and the annulus between the outer surface of thesmaller diameter pipe and inner surface of the larger diameter pipeprovides an annular flow channel. In most concentric pipe designs, thereturning fluid should reabsorb less than 10% of the heat transferred toground. Reference may be had to U.S. Pat. No. 4,574,875 “Heat Exchangerfor Geothermal Heating and Cooling Systems” and US Patent ApplicationPublication No. 20070029066 “Coaxial-Flow Heat Transfer ExchangingStructure for Installation in the Earth and Introducing Turbulence intothe Flow of the Aqueous-Based Heat Transfer Fluid Flowing Along theOuter Flow Channel while Its Cross-Sectional Characteristics ProduceFluid Flows There-along Having Optimal Vortex Characteristics thatOptimize Heat Transfer with the Earth”, which describe prior concentricpiping designs.

The objective of the concentric pipe design is to maximize the heatexchanged between the bulk fluid in the annular flow channel and theearth. As illustrated in FIG. 3, for heat conduction to the earth, theheat must past through three thermal resistances: (1) the resistance 2of the fluid boundary layer separating the bulk fluid and pipe wall; (2)the resistance 4 of the pipe material; and (3) the resistance 6 of thegrout or slurry backfill. Heat loss can also occur between the centerchannel and annular channel, which reduces heat exchange with the earth.This undesirable condition is known as thermal short circuit. Minimizingthermal resistance between the bulk fluid and earth and maximizingthermal resistance between the center channel and the annular channelallows more heat to be exchanged for a given temperature gradientbetween the fluid and the earth. Prior U-tube designs have beenparticularly inadequate in minimizing thermal short circuit while priorconcentric pipe designs have been particularly inadequate in minimizingthermal resistance of the grout and pipe wall.

Vertical, concentric-pipe, ground-loop, heat exchangers are also used asthermal banks for thermal energy storage applications. U-tube designs donot have enough water storage volume or high enough pulse heat transferto make a thermal bank. Ground loops have greater thermal storagecapacity than water tanks and they do not take up any valuable buildingspace. For example, a heat pump can run at night to inject or removeheat from an isolated portion of a ground loop with cheaper electricalrates; then, during the day only a pump circulates fluid from the groundloop thermal bank to handle the heating and cooling loads of thebuilding.

Minimizing the fluid boundary layer thermal resistance 2 requires: (1)maintaining separation between the smaller diameter pipe and the largerdiameter pipe to prevent low flow zones in the annular channel and (2)preventing the development of laminar flow in the annular channel. Thedesign in U.S. Pat. No. 4,574,875 disposes spacers (i.e., centralizers)periodically along the outer surface of the smaller diameter pipe tomaintain alignment between the smaller diameter pipe and the largerdiameter pipe (i.e., to assist in centralizing the smaller pipe withinthe larger pipe). The spacers have projecting spoke-type contacting finswhich are also said to generate an amount of beneficial turbulence inthe annular channel.

The design in US Patent Application Publication No. 20070029066 employsthe method of disposing a helically-wrapped turbulence generator alongthe outer surface of the smaller diameter pipe to generate additionalvorticity. Cost effectively manufacturing such a pipe with helicalfliting disposed along the entire length of the outer surface has provendifficult and such fliting, and pipe, are easily damaged, making theflited pipe difficult to insert into a larger diameter pipe. Attachingthe fliting as a separate piece to a smooth pipe makes the flitingsusceptible to slipping along the outer surface of the pipe, which wouldallow the smaller diameter pipe to come in contact with the largerdiameter pipe, thus creating low flow zones.

Minimizing the thermal resistance 4 of the larger diameter pipe requiresusing a material that: (1) has minimal wall thickness; (2) has enhancedthermal conductivity; (3) has sufficient mechanical strength to preventcollapse during installation; and (4) does not corrode in soil ordegrade in antifreeze environments. Thermoplastic resins such as HDPEand PVC offer sufficient mechanical strength and corrosion resistancebut they also have high thermal wall resistances that would classifythem as thermal insulators. Metal pipe offers very low thermalresistance, but corrosion resistant alloys are very expensive, and theirweight makes them more expensive to ship and more difficult to install.U.S. Pat. No. 4,574,875 prefers the use of plastic for the largerdiameter pipe while US Patent Application Publication No. 20070029066prefers the use of metal or a fluted plastic for the larger diameterpipe. Neither prior design addresses the mechanical strength of thinpipe walls as a function of bore depth.

Minimizing the backfill thermal resistance 6 requires a slurrycomposition that: (1) has enhanced thermal conductivity; (2) has lowpermeability; (3) has sufficiently long set times to allow deployment;(4) is environmentally safe with no organic leachate and less than 1 PPMfor all metals as defined by a TCLP (Toxic Chemical Leaching Procedure);and (5) does not substantially dissipate in geologies with highgroundwater flow. It is common practice to add silica sand to abentonite and water slurry to enhance thermal conductivity toapproximately 1.4 Btu/hr-ft-° F. Reference may be had to US PatentApplication Publication No. 20070125274 “Thermally Conductive Grout forGeothermal Heat Pump Systems”, which describes the use of graphiteparticles, ranging from 10 to 1000 microns in size, added to the slurryin concentrations from 2 to 25% by weight to produce a backfill withthermal conductivity greater than 4 W/m-K (2.3 Btu/hr-ft-° F.) that haslower permeability. The prior art is inadequate in providing detailsspecifying a backfill composition that would be pumpable, would enablesufficiently long set times for deployment, and would resist dissipationdue to high ground water flow rates. U.S. Pat. No. 4,574,875 does notaddress backfill composition and US Patent Application Publication No.20070029066 prefers the use of thermally conductive cement but is notspecific in backfill mixture composition, nor does it addresspermeability, environmental safety or dissipation.

Minimizing thermal short circuit requires that the center channel besufficiently insulated from the annular channel to prevent significantheat flow between the channels. The design in U.S. Pat. No. 4,574,875offers no solution while the design in US Patent Application PublicationNo. 20070029066 prefers relying on laminar flow in the center channel orusing an insulating gas within the smaller diameter pipe, which are bothimpractical solutions to implement.

The designs in both U.S. Pat. No. 4,574,875 and US Patent ApplicationPublication No. 20070029066 also fail to provide solutions that: (1)minimize pressure drop across the system; (2) prevent blockage of thecenter channel outlet; and (3) facilitate installation.

In addition to the above, further shortcomings and problems withexisting concentric ground exchange assemblies arise due to the designof the heat transfer fluid supply and return headers currently used inthese systems. A concentric ground exchange assembly 102 of a typeheretofore known in the art is depicted in FIG. 8. The prior artassembly 102 comprises: an elongate outer casing string 104 whichextends into the ground; a smaller diameter elongate inner pipe string106 which extends downwardly inside the casing 104 such that a returnflow annulus 108 is provided between the exterior of the inner conduit106 and the interior wall of the casing 104; a plurality of centralizingelements or structures (not shown) which are positioned at intervalsalong the exterior of the inner conduit 106 for maintaining the innerconduit 106 in a substantially concentric alignment within the interiorof the casing 104; an optional turbulence generating structure 110(i.e., helical fliting) extending along the exterior of the innerconduit 106 for producing flow turbulence within the return flow annulus108; and a heat transfer fluid supply and return header 112 secured atthe upper end of the concentric ground exchange assembly 102.

The prior art concentric ground exchange assembly 102 will typically beinstalled in a vertical borehole which has been drilled to a depth in arange of from about 100 to about 500 feet and has a diameter in therange of from about five to about eight inches. As mentioned above, theconcentric exchange assembly 102 is inserted into the borehole and theborehole is typically backfilled with a grout slurry composition whichhardens to form a substantially impermeable grout barrier. The groutbarrier prevents or at least reduces the vertical movement of groundwater within the borehole and provides a heat transfer bridge betweenthe exterior of the casing and the interior wall of the borehole.

The heat transfer working fluid employed in the concentric groundexchange assembly 102 will typically be either water or a mixture ofwater and antifreeze. Again, although refrigerants such as FREON® orother materials can alternatively be used, such materials are typicallynot employed due to the cost of materials and the danger of aquifercontamination.

During the operation of the concentric exchange assembly 102, theworking fluid is delivered from a fluid supply line (not shown) to theinlet port 114 of the supply and return header 112. The supply linetypically extends horizontally underground and is therefore oftenreferred to as a “lateral.” Moreover, as a consequence of thesubstantially horizontal orientation of the supply lateral, it isnecessary that the header inlet include an elbow 115 which directs thefluid supplied to the inlet port 114 downwardly through the innerconduit 106. The fluid flows out of the lower end portion of the innerconduit 106 and is then directed upwardly through the return flowannulus 108 provided between the inner wall of the casing 104 and theexterior of the inner conduit 106. As the working fluid flows upwardlythrough the return flow annulus 108, the fluid is either heated orcooled by heat transfer with the earth and then discharged to a returnline (lateral) (not shown) connected to the header discharge port 116.

Unfortunately, the supply and return header 112 used in the prior artconcentric ground exchange assembly 102 has presented numerous problemsand difficulties. The prior art supply and return header 112 comprises:a tall vertical inlet conduit 118 which includes the inlet elbow 115 atthe top thereof and extends downwardly to the upper end of the innerconduit 106; a horizontally oriented connector 122 extending from theelbow 115 for connection of the working fluid supply line (lateral); anouter housing 124 provided around the lower exterior of the inletconduit 118, below the elbow 115, for receiving fluid from the returnflow annulus 108; a horizontal connector 126 extending from the outerhousing 124 for attachment of the working fluid return line (lateral); aflange 128 provided at the lower end of the supply and return header 112for attaching the header 112 to a corresponding flange 130 which must beinstalled on the upper end of the casing 104; a plurality of (typically4) bolts 132 and associated nuts and washers for securing the headerflange 128 on the casing flange 130; and a flange gasket 134 positionedbetween the header flange 128 and the casing flange 130.

In addition to other shortcomings, the underground flange connectionrequired by the prior art supply and return header 112 is susceptible tosignificant leakage and other problems resulting from: (a) thermalcontraction and expansion of the header material (typically high densitypolyethylene), (b) deterioration of the gasket 134 and/or the bolts 132and associated nuts and washers, (c) the application of insufficient orexcessive torque to the flange bolts 132 during installation, (d) torquecreated by surface vehicular loads, and/or (e) the loosening of the nutsand bolts over time. Moreover, due to its excessive height required foraccommodating the inlet elbow configuration 115, the installation of theprior art supply and return header 112 requires a trenching depth ofbetween 5 to 6 feet or more.

SUMMARY OF THE INVENTION

The present invention satisfies the needs and alleviates the problemsdiscussed above. The invention provides a system, equipment, devices,compositions, and a method for subterranean ground heat exchange whichminimize both (a) thermal short circuit and (b) the three thermalresistances between the working fluid and the earth, while alsoproviding solutions to other problems not addressed in prior art.

In addition, the present invention provides an improved supply andreturn header and an improved concentric ground exchange assembly havingthe inventive supply and return header installed on the upper endthereof. The inventive supply and return header satisfies the needs andalleviates the problems discussed above by: (a) eliminating the flangeconnection to the casing required by the prior art header; (b) replacingthe flange connection with a header skirt or collar which is chemicallyfused directly to the upper end of the casing; and (c) significantlyreducing the height of the header to as little as ⅓ or less of theheight of the prior art structure so that significantly less trenchingand digging is required when installing the inventive system. Theinventive supply and return header therefore has a much greater life, ismuch more reliable, is much better able to withstand surface vehicularloads, and is less costly and much easier to install.

In one aspect, there is provided a system, assembly, and method for lowthermal resistance ground heat exchange between a working fluid and theearth. The system can be used for the heating and cooling of buildingspaces and can provide thermal energy storage. The system can be coupledto a heat pump or provide direct cooling by being coupled to a fluidcooler. The system preferably comprises: a low thermal resistancecasing; a ribbed drop tube inserted into the casing; standoffs disposedon the exterior of the drop tube to maintain separation between the droptube and the casing; an end cap to seal the bottom of the casing; and aheader with supply and return ports. The system is preferably installedusing an inventive, thermally conductive grout composition.

In another aspect, there is provided an apparatus and system forheating, cooling, and thermal energy storage using low thermalresistance ground heat exchange, the apparatus and system preferablycomprising:

-   -   (a) an outer cylindrical, fiber-resin composite casing which        provides a pressure boundary;    -   (b) a smaller diameter cylindrical drop tube arranged        substantially concentrically within the cylindrical casing to        create a center channel and an annular channel whereby the cross        sectional area of the annular channel is preferably greater than        cross sectional area of the center channel;    -   (c) the interior surface of the drop tube being smooth to        minimize pressure drop and heat transfer along the center        channel;    -   (d) the exterior surface of the drop tube being ribbed to create        flow vortices and induce flow turbulence in the annulus to        disrupt the thermal boundary layer on the interior casing        surface to increase heat transfer to the earth;    -   (e) standoffs disposed along the outer surface of the drop tube        such that the drop tube is substantially centralized in the        casing, heat transfer is increased, and pressure drop is        reduced;    -   (f) an end cap which seals the bottom of the casing; and    -   (g) a header to seal the top of the casing, the header providing        a port to connect the center channel to a supply line and a port        to connect the annular channel to a return line.

In another aspect, there is provided a conduit for conductive heattransfer comprising a composite wall structure formed from athermosetting plastic composition and a reinforcing fiber material,wherein: (a) a flow passageway extends through the composite wallstructure; (b) the reinforcing fiber material is fiberglass, carbonfiber, aramid fiber, or a combination thereof; (c) the thermosettingplastic composition from which the composite wall structure is formedincludes an amount of a thermal conductivity enhancing additive of atleast 1.5% by weight based upon the total weight of the thermosettingplastic composition; and (d) the thermal conductivity enhancing additiveis aluminum flake, aluminum powder, aluminum oxide, aluminum nitride,graphite, boron nitride, silicon carbide, Raney nickel, silver-coatednickel, silver-coated copper, or a combination thereof.

The inventive conduit can be used for conductive heat transfer and/orother purposes in numerous types of applications, exchangers, andsystems. The inventive conduit is particularly well suited for use as acasing for any type of ground heat exchange system including, but notlimited to, both U-tube and concentric exchanger configurations, as wellas ground loops having any type of vertical, horizontal, angled, ordeviated orientation.

In another aspect, there is provided a method of forming a conduit forconductive heat transfer. The method comprises the steps of: (a)applying a thermosetting epoxy composition to a continuous fibermaterial and (b) winding the continuous fiber material around a mandrel.The continuous fiber material is preferably fiberglass, carbon fiber,aramid fiber, or a combination thereof. The thermosetting epoxycomposition includes an amount of medium grade aluminum powder in therange of from about 1.5% to about 8% by weight (more preferably at least2% by weight) based upon the total weight of the thermosetting epoxycomposition.

In another aspect, there is provided a method of subterranean groundheat exchange comprising the step of flowing a fluid medium through anunderground casing such that thermal energy is conducted through a wallof the casing between the fluid medium and an underground environmentsurrounding the casing. The wall of the casing is a composite wallformed from a thermosetting plastic composition and a reinforcing fibermaterial wherein: (a) the reinforcing fiber material is fiberglass,carbon fiber, aramid fiber, or a combination thereof; (b) thethermosetting plastic composition from which the composite wall isformed includes an amount of a thermal conductivity enhancing additiveof at least 1.5% by weight based upon the total weight of thethermosetting plastic composition; and (c) the thermal conductivityenhancing additive is aluminum flake, aluminum powder, aluminum oxide,aluminum nitride, graphite, boron nitride, silicon carbide, Raneynickel, silver-coated nickel, silver-coated copper, or a combinationthereof.

In another aspect, there is provided a method of subterranean groundheat exchange comprising the step of flowing a fluid medium through aflow annulus in an underground casing such that thermal energy isconducted through a wall of the casing between the fluid medium and anunderground environment outside of the casing. The flow annulus isformed between an interior wall of the casing and an internal conduitwhich extends into the casing. The internal conduit has a series ofdiscrete, spaced-apart, radial ribs such that the series of ribs extendsalong an exterior of the internal conduit and such that the radial ribsproject into the flow annulus toward the interior wall of the casing.

In another aspect, there is provided an apparatus for subterraneanground heat exchange comprising: (a) a casing which extends underground;(b) an internal delivery conduit extending into the casing fordelivering a fluid medium to a distal end portion of the internaldelivery conduit such that the fluid medium will flow from the distalend portion of the internal delivery conduit into a flow annulus formedbetween the internal delivery conduit and an interior wall of thecasing; and (c) a series of discrete, spaced-apart, radial ribs whereinthe series of ribs extends along an exterior of the internal deliveryconduit such that the radial ribs project into the flow annulus.

In another aspect, the inventive system is installed using a novelbackfill slurry composition. The composition preferably comprisesbentonite (or cement), graphite and water such that the compositionpreferably has a solids content of at least 25% solids by weight. Theslurry is used to backfill a borehole and provides a thermalcommunication path between the outer casing and the wall of the borehole(i.e., between the outer casing and the subterranean ground environmentsurrounding the casing). The graphite content is preferably an amountsufficient to provide a thermal conductivity of at least 3.0Btu/hr-ft-F.

In another aspect, in order to further promote conductive heat transfer,it is also preferred concerning the novel casing used in the low thermalresistance ground heat exchange system that the casing be a thin-walledcasing wherein : (a) the casing is preferably formed by embedding fiberor surrounding fiber in a thermosetting resin by applying thethermosetting resin composition to the fiber (e.g., by wetting the fiberin a thermosetting resin bath) and then winding the wetted fiber arounda mandrel; (b) the large diameter cylindrical casing has a ratio ofouter diameter to inner diameter that is less than 1.1; (c) the casingend cap is equipped with holes to allow a rope or wire to be threadedthrough the end cap to facilitate lowering the casing assembly into thebore; (d) the end cap is equipped with a check valve and cement floatshoe to facilitate grouting by pumping down the casing; (e) flow portswith cumulative cross sectional area greater than cross sectional areaof center channel are provided in or at a distal end portion of thecenter channel near the center channel outlet; and/or (f) the outercasing is constructed in segments of 15-30 feet with male threads at oneend and female threads at the other end. If desired, the thermosettingresin composition can optional also include other additives such as,e.g., a standard wetting and dispersing agent to facilitate dispersionof the resin fillers and wetting out of the fiber and/or an air releaseadditive to reduce entrapped air bubbles to increase heat transfer.

In another aspect, the backfill grout slurry dry base preferablycomprises (a) at least 70 parts by weight (more preferably from about 75to about 85 parts by weight) natural flake graphite, the naturalgraphite preferably having a particle size of less than 200 mesh, and(b) from about 15 to about 30 parts by weight (more preferably fromabout 25 to about 15 parts by weight) bentonite or Portland cement.However, by way of example, one alternative backfill slurry dry mixcomposition can comprise at least 70% by volume amorphous graphite,synthetic graphite, or coke and from about 2% to about 5% by volumecalcium sulfate with the remaining volume being made up of sodiumbentonite.

In another aspect, there is provided a grout slurry composition forconductive heat transfer applications, the grout slurry compositionconsisting essentially of: (a) a grout dry base composition consistingessentially of from about 70 to about 85 parts by weight natural flakegraphite and from 30 to about 15 parts by weight of bentonite, Portlandcement, or a combination thereof, and (b) an amount of water in therange of from about 8 to about 12 gallons for each 50 pounds of thegrout dry base composition. The natural flake graphite preferably has aparticle size effective for causing the natural flake graphite to remainin suspension in the grout slurry composition during use without anydispersant or other suspension assisting agent being present in thegrout slurry composition.

In another aspect, it is also preferred concerning the novel low thermalresistance ground heat exchange system that: (a) the smaller diametercylindrical drop tube have a ratio of outer diameter to inner diameterof at least 1.2 to mitigate thermal short circuiting; (b) the standoffsemployed each have three loops constructed from one continuous wire andthat the standoffs be open so that they can be clipped onto the droptube; (c) an ultrasonic horn be pulled up the casing to instantly setthe grout in the wellbore; and/or (d) the graphite enhanced cementmixture be used to provide structural support for the building piers andthermal conductivity for the ground heat exchanger.

The larger diameter cylindrical casing formed by the present inventionminimizes thermal resistance and has sufficient crush and bursttolerance to be installed in depths of up to 500 ft or more. The novelcylindrical casing also resists corrosion when buried in the ground orwhen exposed to anti-freeze. In addition, the novel cylindrical casingpreferably weighs less than 1 pound per foot and is preferably equippedwith male and female threads for easy field assembly. Further, the abovedescribed thermosetting resin composition additives increase the thermalconductivity of the cylindrical outer casing. Also, the hole provided inthe casing end cap allows a rope or wire to be threaded therethrough forlowering the assembled casing into the borehole. However the hole isformed and positioned such that it does not prevent the end cap fromsealing the end of the casing.

In another aspect there is provided a grout slurry composition forconductive heat transfer applications comprising: (a) a dry basepremixed composition consisting of from about 70 to 85 parts by weightnatural flake graphite and from about 30 to about 15 parts by weightbentonite and (b) water in an amount of not more than 12 gallons foreach 50 pounds of the dry base premixed composition. The natural flakegraphite has a particle size of not greater than 200 mesh. The dry basepremixed composition and water are preferably present in concentrationssuch that the grout slurry composition has a solids content of at least35% by weight based upon the total weight of the grout slurrycomposition. In addition, the grout slurry composition preferably alsocomprises a thinning agent present in an amount such that the groutslurry composition is pumpable.

In another aspect there is provided a grout slurry compositioncomprising: from about 70 to about 85 parts by weight natural flakegraphite; from about 30 to about 15 parts by weight of bentonite; waterin an amount effective such that the grout slurry composition has asolids content of at least 30% by weight based on the total weight ofthe grout slurry composition; and a thinning agent. The thinning agentis preferably sodium chloride, potassium chloride, or a combinationthereof and is present in the grout slurry composition in an amounteffective such that the grout slurry composition is pumpable. The groutslurry composition more preferably has a solids content of at least 35%by weight solids and most preferably has a solids content of at least40% by weight solids.

In another aspect there is provided a method of installing a casing of asubterranean ground heat exchange system. The method comprises the stepsof: (a) placing the casing in a borehole; (b) forming a treated water byadding a thinning agent to water; (c) adding the treated water to a drybase composition to form a grout slurry; and (d) placing the groutslurry in an annulus between an interior wall of the borehole and anouter wall of the casing. The dry base composition consists essentiallyof from about 70 to about 85 parts by weight natural flake graphite andfrom 30 to about 15 parts by weight bentonite. The treated water isadded to the dry base composition in an amount such that the groutslurry formed in step (c) has a solids content of at least 30% by weightbased on the total weight of the grout slurry. The thinning agent isadded to the water in step (b) in an amount effective to allow the groutslurry to be placed in the annulus in accordance with step (d) bypumping. The grout slurry more preferably has a solids content of atleast 35% by weight and more preferably has a solids content of at least40% by weight. The thinning agent is preferably sodium chloride,potassium chloride, or a combination thereof. In one embodiment, thegrout slurry formed in step (c) and placed in the annulus by pumping instep (d) consists solely of the dry base composition, the water, and thethinning agent.

In another aspect there is provided an improvement for a subterraneanground heat exchange apparatus of the type comprising a casing extendingunderground, an internal delivery conduit extending into the casing, anda supply and return header mounted at an upper end of the casing forsupplying a heat transfer fluid into the internal delivery conduit andfor receiving the heat transfer fluid as it returns to the supply andreturn header via a return flow annulus between the internal deliveryconduit and an interior wall of the casing. The improvement comprises aninventive supply and return header comprising: a housing having a closedtop and a vertically extending outer wall which surrounds an interior ofthe housing, the closed top defining an upper end of the supply andreturn header; a supply port for the heat transfer fluid providedthrough the vertically extending outer wall; a return port for the heattransfer fluid provided through the vertically extending outer wall; andan interior supply conduit. The interior supply conduit includes a firstportion thereof which extends into the interior of the housing from thesupply port and has a bend which is positioned in the interior of thehousing for directing the heat transfer fluid downwardly. The interiorsupply conduit also includes a second portion thereof which extendsdownwardly from the bend for delivering the heat transfer fluid to theinternal delivery conduit of the ground heat exchange apparatus.

In another aspect, the inventive supply and return header used in theinventive improved ground heat exchange apparatus preferably has aheight effective such that, when the inventive header is mounted on theupper end of the casing, the upper end of the supply and return headerwill not be more than 10 inches (more preferably not more than 8 inches,even more preferably not more than 7 inches, and most preferably notmore than 6 inches) above the upper end of the casing.

In another aspect, there is provided a rubber or plastic drop tube thathas ribs disposed along its outer surface to create flow vortices andinduce flow turbulence in the annulus and cause the thermal boundarylayer to restart along the casing inner wall.

In another aspect, there is provided a drop tube that resists corrosionand decomposition when exposed to antifreeze.

In another aspect, there is provided a drop tube that has a thermalconductivity value of less than 0.2 Btu/hr-ft-° F. to reduce thermalshort circuiting.

In another aspect, there is provided a plastic or corrosion resistantmetal standoff to maintain alignment between the drop tube and innercasing wall to prevent low flow zones.

In another aspect, there is provided a procedure wherein standoffs areplaced in the drop tube corrugation valleys to prevent vertical movementalong the drop tube.

In another aspect, there is provided a drop tube with a smooth innersurface to minimize pressure drop along the center channel.

In another aspect, there is provided a drop tube that has an outerdiameter such that the cross sectional area of the annulus is preferablyequal to or greater than that of the inner channel in order to minimizepressure drop in the annulus.

In another aspect, there is provided a drop tube having holes in or atthe bottom end portion thereof to provide flow channels should thebottom end of the drop tube come in contact with the casing end cap. Theholes preferably have a combined area equal to or greater than thecenter channel area to minimize pressure drop.

In another aspect, there is provided a slurry backfill composition thathas thermal conductivity of preferably greater than 3.0 Btu/hr-F andpermeability of preferably less than 1×10⁻⁷ cm/s.

In another aspect, there is provided a grout composition that has a settime greater than one hour and that does not significantly dissipate ingeologies with significant ground water flow.

In another aspect, there is provided a grout composition that will notcontaminate the environment.

Further aspects, features and advantages of the present invention willbe apparent to those of ordinary skill in the art upon examining theaccompanying drawings and upon reading the following DetailedDescription of the Preferred Embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical, partially cut-away view of an embodiment 10 of amechanical assembly including an outer casing 12, an end cap 14, aribbed drop tube 16 with standoffs 18 and flow ports 20, and a header 22for connecting the tube center channel 24 to a supply line andconnecting the annular channel 26 between the tube 16 and the inner wall28 of the casing 12 to a return line.

FIG. 2 is a cutaway cross-sectional view of the mechanical assembly 10as seen from perspective 2-2 shown in FIG. 1.

FIG. 3 illustrates the thermal resistance (fluid boundary layer 2,casing 4 and slurry backfill 6) between the bulk fluid 5 in the annularchannel of a concentric geothermal exchange assembly and the earth 8.

FIG. 4 illustrates an embodiment of an inventive process for forming theinventive casing 12 wherein fibers 30 wetted with thermosetting resincomposition 32 are wound on mandrel 34.

FIG. 5 shows an embodiment of the inventive casing 12 with male threads36 and female threads 38.

FIG. 6 is a partially cutaway sectional side view illustrating themechanical assembly 10 installed underground in a vertical borehole 11.FIG. 6 also illustrates the drop tube 16 having a ribbed outer surface40, a smooth inner surface 42, and standoffs 18 attached between ribs44.

FIG. 7 shows an embodiment of an inventive standoff 18 used inaccordance with the present invention.

FIGS. 8A and 8B are exterior and a cutaway elevation views of aconcentric ground heat exchange assembly 102 using a prior art supplyand return header 112.

FIG. 9 is a schematic elevational cutaway side view of the upper end ofan inventive concentric ground heat exchange assembly 200 having anembodiment 202 of the inventive supply and return header installed onthe upper end thereof.

FIG. 10 is an exterior elevational side view of the inventive supply andreturn header 202.

FIG. 11 is a bottom perspective view of a socket-to-thread adaptor 204used for attaching the inventive supply and return header 202 to theupper end of the ground heat exchange assembly casing 206.

FIG. 12 is a perspective view of the inventive supply and return header202 having the socket-to-thread adaptor 204 installed thereon.

FIG. 13 is an upper perspective view showing the socket-to-threadadaptor 204 received on and chemically bonded to the upper end of thecasing 206.

FIG. 14 is a cutaway vertical view of a concentric ground heat exchangeassembly using an alternative embodiment of the inventive supply andreturn header 202.

FIG. 15 is a cutaway vertical view of an alternative cast embodiment 302of the inventive supply and return header.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A bore 11 of desired size, preferably measuring between about 5 to about8 inches in diameter, is drilled vertically into the earth to a depthbetween from about 100 to about 500⁺ feet. The inventive mechanicalassembly 10 shown in FIGS. 1 and 2 is installed in the borehole 11 andthen the borehole 11 is backfilled with an aqueous grout slurry 15preferably comprising natural flake graphite and a binding material suchas bentonite or Portland cement. Alternatively, by way of example,amorphous graphite, bentonite, and calcium sulfate could be used. As isillustrated, the pressure boundary of the mechanical assembly 10preferably comprises an outer (i.e., a larger diameter) cylindricalcasing 12 capped at the bottom by an end cap 14 and at the top by aheader 22 equipped with a supply port 23 and a return port 25.

A ribbed drop tube 16 of smaller diameter is inserted inside the casing12 and aligned concentrically with (i.e., substantially centralized in)the casing 12 to create a center flow channel 24 and an annular flowchannel 26. The supply port 23 connects the center flow channel 24 to asupply line and the return port 25 connects the annular flow channel 26to a return line. The working fluid, which is preferably water or awater and antifreeze mixture, enters the mechanical assembly from thesupply line, through the supply port 23, and travels down the centerchannel 24. The working fluid exits the center channel 24 through thedrop tube outlet 21 at the lower end (i.e., distal end) of the drop tube16 and through the flow ports 20. The flow ports 20 provide analternative flow path in case the drop tube bottom outlet 21 comes incontact with the bottom of the casing. The working fluid then travelsupward through the annular flow channel 26, then through the return port25, and exits the mechanical assembly 10 to the return line.

Heat is exchanged between the earth 8 (i.e., the underground environmentsurrounding the casing 12) and the bulk fluid 5 flowing in the annularchannel if a temperature gradient exists between the fluid 5 and earth8. The bulk fluid 5 rejects heat (i.e., is cooled) when the temperatureof the bulk fluid 5 is greater than the temperature of the earth 8. Thebulk fluid 5 absorbs heat (i.e., is heated) when the temperature of thebulk fluid 5 is less than the temperature of the earth 8.

As illustrated in FIG. 3, the heat exchanged between the earth 8 and thebulk fluid 5 flowing in the annular channel 26 must travel through threethermal resistances: the fluid boundary layer resistance 2; the casingresistance 4; and the slurry backfill (grout) resistance 6. To rejectheat from the bulk fluid 5, the heat flows through the fluid boundarylayer to the casing inner wall 28, then through the casing 12 and thenthrough the slurry backfill 15. For the bulk fluid 5 to absorb heat, theheat flows through the slurry backfill 15, then through the casing 12and then from the casing inner wall 46 to the bulk fluid 5 through thefluid boundary layer. The present invention provides practical materialsand methods to reduce all three thermal resistances, thereby allowinggreater heat exchange per unit length of the mechanical assembly 10 fora given temperature gradient. The present invention also provides apractical method to reduce thermal short circuiting between the centerchannel 24 and annular channel 26.

As illustrated in FIG. 4, the present invention provides a novel conduitwhich is well suited for use as the outer cylindrical casing 12 for thesystem pressure boundary. However, the inventive conduit 12 is also wellsuited for use in other conductive heat transfer applications and can beformed in any desired cross-sectional shape. The inventive casing 12 ispreferably formed by embedding fiber 30, or surrounding fiber 30, in athermosetting resin by first wetting the fiber in a thermosetting resinbath 32, or otherwise applying the thermosetting plastic composition tothe fiber 30, and then winding the wetted fiber 30 around a mandrel 34.The fibers 30, preferably made of glass, carbon or aramid (mostpreferably fiberglass direct-draw roving), impart directional strengthto the material while the thermosetting resin 32 bonds the fibers 30together and transfers stress between the fibers 30. The thermosettingplastic composition 32, which is preferably comprised of an epoxy baseresin and an amine or anhydride curing agent, polymerizes to apermanently solid state upon the application of heat. An example of apreferred epoxy system is a two-part system comprising (a) an epoxyresin (e.g., FR-240 Epoxy Part A from Smith Fibercast or Dow DER 383diglycidylether of bisphenol A) and (b) methylene dianiline or othercuring agent (e.g., FR-204 Epoxy Part B from Smith Fibercast).

The thermosetting plastic composition 32 used for forming the inventivecasing preferably comprises at least 1.5% (more preferably at least 2%)by weight, based on the total weight of the thermosetting plasticcomposition, of at least one additive such as graphite, aluminum flake,aluminum powder, aluminum oxide, aluminum nitride, boron nitride,silicon carbide, Raney nickel, silver-coated nickel or silver-coatedcopper to enhance thermal conductivity. As will be understood by thosein the art, the resin mixture may also optionally contain other typicaladditives such as, for example, standard wetting and dispersing agentsto facilitate dispersion of the resin and fillers and wetting out of thefiber and air release agents to reduce entrapped air bubbles.

The thermal conductivity additive used in forming the inventive casingconduit 12 is preferably aluminum powder. The thermal conductivityadditive is most preferably a medium grade, dedusted, leafing aluminumflake powder having a ⁺325 mesh retention of not more than 2%. Although,pound for pound, carbon is more thermally conductive, it has beendiscovered in accordance with the present invention that the filamentwinding epoxy loading characteristics of aluminum powder, particularlywhen using the preferred medium grade powder, are of such a surprisinglysuperior and unexpected nature that much higher loadings of the aluminumpowder providing greater thermal conductivities are achieved. Moreover,even at such high loadings, a desirably thin-walled yet unexpectedlystrong pipe is produced and the viscosity of the epoxy system remainsrelatively low, thus speeding the pipe manufacturing process. Further,the aluminum powder additive is also readily available at reasonablecost.

The amount of the aluminum powder additive used in the thermosettingepoxy composition will preferably be in the range of from about 1.5 toabout 8% by weight (more preferably at least 2% and most preferablyabout 2.6%) based on the total weight of the thermosetting epoxycomposition 32. In addition, the thermosetting epoxy system compositionwill preferably further comprise an amount of an epoxy resin (i.e., theepoxy system Part A component) in the range of from about 77.8% to about67.2% by weight and an amount of a curing agent (i.e., the epoxy systemPart B component) in the range of from about 20.7% to about 17.8% byweight, all based on the total weight of the thermosetting epoxycomposition.

In forming the casing 12, the wetted fibers 30 are preferably wound toproduce a casing or other conduit 12 with a ratio of outer diameter toinner diameter of less than 1.1 in order to minimize thermal resistance,while still maintaining sufficient crush pressure (75 psi or more) andburst pressure (300 psi or more) for (a) vertical deployment to a depthof up to 300 ft. in conjunction with the use of the inventive backfillslurry preferably blended to a specific gravity of 1.4 or less and (b)vertical deployment up to 300 to 400 feet, or even up to 500⁺ feet, whenpreferably using a backfill slurry with a specific gravity of 1.3 orless. The inventive casing is preferably less than 1 pound per foot fordiameters up to 3.5 inches and does not corrode in soil or antifreezeenvironments.

As shown in FIG. 5, the casing 12 is preferably fashioned intomanageable segments of about 15 to about 30 feet in length with one endhaving male threads (spigot end) 36 and the other end having femalethreads (bell end) 38, making the casing 12 easy to assemble. An epoxyadhesive is preferably placed on the male threads 36 during assembly toensure a good mechanical seal.

Concerning the wall thickness of the inventive conduit 12, the wettedfiber 30 is more preferably wound on the mandrel 34 such that the ratioof the outside diameter to the inside diameter of the conduit 12 is notmore than 1.055, most preferably not more than 1.04, with a collapsepressure of at least 75 psig. When used as a casing, the dimensions ofthe inventive conduit 12 will preferably be such that (a) the interiorflow passageway 24 of the conduit 12 will be at least 9 inches incircumference and will more preferably be about 3.366 inches or more indiameter based upon the use of a 3.366⁺ inch diameter forming mandrel 34and (b) the conduit wall 52 will have a thickness of not more than 0.12inch, more preferably not more than 0.085 inch and most preferably notmore than 0.065 inch, while maintaining a collapse pressure of at least75 psig.

As shown in FIG. 1, the present invention provides an end cap 14 that ispreferably pressure molded from chopped fiber and thermosetting resin. Ahole 54 through the end cap 14, preferably measuring approximately 0.5inches in diameter, is provided to allow connection for a rope or wire53 to facilitate lowering the casing assembly into the borehole 11. Theend cap hole 54 is provided in such a manner as to not interfere withthe casing seal provided by the end cap 14. The end cap 14 is preferablysecured to the bottom casing segment using epoxy adhesive.

As illustrated in FIG. 6, the present invention provides an innercylindrical ribbed drop tube 16 that is preferably made from HDPE, PVC,or EPDM (i.e., a terpolymer elastomer produced from ethylene-propylenediene monomer) or any other rubber or plastic that is chemicallyresistant to common antifreeze additives such as methanol, ethanol,ethylene glycol and propylene glycol and that preferably has a thermalconductivity value less than 0.2 Btu/hr-ft-F. The drop tube 16 is asmaller diameter conduit which includes a series 58 of discrete, spacedapart, radial ribs 44, said series 58 of ribs 44 extending along theexterior of the drop tube 16 such that the radial ribs 44 project intothe flow annulus 26 formed between the exterior of the drop tube 16 andthe interior wall 28 of the casing 12. The radial ribs 44 are preferablycircular in shape as shown in FIG. 2 and are preferably sized and spacedapart as shown in FIG. 1 such that, for each adjacent pair of ribs 44,the ratio of the peak-to-peak rib spacing 60 to the radial peak ribheight 62 is in the range of from about 1.2:1 to about 3:1.

The ratio of the drop tube outer diameter (as measured between the ribs44 at the point 64 of minimum tube thickness) to drop tube innerdiameter will preferably be at least 1.2 to reduce thermal shortcircuiting between the center fluid delivery channel 24 of the drop tube16 and the annular channel 26. The drop tube outer diameter ispreferably sized such that, at the point 64 of the minimum tubethickness, the cross sectional area of the annular flow channel 26 isgreater than the cross sectional area of the center channel 24 in orderto minimize pressure drop along the annular channel 26.

The drop tube 16 is preferably a corrugated plastic or rubber tube. Suchtubing is typically available on spools which makes it particularlyconvenient for installation and use in the inventive application.Because of the length of the tubing, few connections are needed for agiven application. However, where necessary, sections of the corrugatedtubing can be connected together mechanically using, e.g., plastic orstainless steel barbed connectors that are reinforced by applying bandson either end of the connection.

The drop tube inner surface 42 will preferably be smooth to reducepressure drop along the center channel 24. The ribs 44 are preferablydisposed along the outer surface of the drop tube 16 perpendicular tothe longitudinal axis 68 of the drop tube 16 to create flow vortices 70and induce flow turbulence in the annulus 26 and cause the thermalboundary layer to restart along the casing inner wall 28. However, thenature of the ribs 44 is such that the same turbulent flow conditionswill not exist adjacent the drop tube 16 in the valleys 72 between theribs, thus further reducing thermal short circuiting by not promotingthe disruption of the fluid boundary layer on the exterior of the droptube 16 at the points 64 of minimum tube thickness. The ribs 44 willpreferably have a pitch in the range of from about 80 to about 90degrees in order to assist in preventing the ribbed tube 16 frombecoming deformed during shipping, packing, and installation.

As seen in FIG. 1, flow ports 20 are preferably provided at the lowerend of the drop tube 16 to allow the fluid to exit the center channel 24should the drop tube outlet 21 become blocked by the casing bottom. Thismay occur as the drop tube 16 elongates over time. The cumulative areaof the flow ports 20 will preferably be greater than the cross sectionalarea of the center channel 24 to minimize pressure drop across the flowports 20. The flow ports may be drilled through the side wall at thebottom portion of the drop tube 16 near the lower end 21. Alternatively,an insert 76 with flow ports 20 can be inserted into the drop tubeoutlet 21.

As illustrated in FIGS. 2, 6, and 7, the present invention providesplastic or corrosion resistant metal standoffs 18 that are clipped ontothe exterior of the drop tube 16 between the ribs 44, preferablyproviding three points of contact between the drop tube 16 and the innerwall 28 of the outer casing. The ribs 44 prevent vertical movement ofthe clips 18 along the drop tube 16. The standoff contacts 78 arepreferably in the shape of a loop, providing a rounded surface tominimize scoring of the inner casing wall 28 during insertion of thedrop tube into the casing. The standoff 18 preferably has at least threecontact loops 78.

The present invention also provides a thermally conductive, lowpermeability slurry backfill that is made from environmentally safematerials. The slurry is preferably made by mixing a dry base mixturewith water in sufficient quantity to produce a slurry with at least 25%solids by weight. The dry base mixture preferably comprises, and morepreferably consists essentially of or consists of, (a) natural flakegraphite, amorphous graphite, synthetic graphite, or coke, and (b)bentonite or Portland cement.

Graphite enhances the conductivity of the slurry and the insolublegraphite particles provide a barrier to slow the vertical flow of water.Sodium bentonite, which is environmentally friendly clay, hydrates tofill the interstitial spaces of the graphite particles, further reducingpermeability to less than 1×10⁻⁷ cm/s. When using amorphous graphite,calcium sulfate, one of the more common minerals in sedimentaryenvironments, preferably replaces 2-5% of sodium bentonite in the drymixture to prevent dissipation of the backfill when the backfill isdeployed in geologies with significant ground water flow.

In the case of amorphous graphite, the dry mix preferably comprises atleast 70% amorphous graphite by volume with bentonite making up theremaining volume. Amorphous graphite, which is formed in the earth underintense heat and pressure, is inorganic, insoluble, and is virtuallyfree of polyaromatics and metals.

In a particularly preferred embodiment, the inventive grout slurrycomprises, and more preferably consists essentially of, (a) a dry basemix including, and more preferably consisting essentially of, from about70 to about 85 (most preferably from about 75 to about 80) parts byweight (pbw) natural flake graphite and from about 30 to about 15 (mostpreferably from about 25 to about 20) pbw bentonite, Portland cement, ora combination thereof and (b) an amount of water in the range of fromabout 8 to about 12 (most preferably from about 9 to about 11) gallonsper each 50 pounds of the dry mix. The resulting slurry has a solidsconcentration in the range of from about 45% to about 30% (mostpreferably from about 40% to about 35%) by weight. If high ground waterflow is present, the composition of inventive grout slurry can bechanged to also include up to 25 pounds of sands and/or up to 5 poundsof gypsum per each 50 pounds of the dry mix.

To assist in maintaining the flake graphite in suspension without theuse of any dispersants, polymers, or other suspension assisting agentswhich can detract from the desirable thermal properties of the grout,and to also permit the incorporation of higher concentrations ofgraphite for significantly higher thermal conductivity, the particlesize of the flake graphite used in the preferred grout slurrycomposition is preferably not greater than 200 mesh (0.074 mm). Inaddition, the grout slurry is preferably pumped into the borehole 11around the outside of the casing 12 using a nonshearing pump (e.g., apositive displacement or diagram pump) and a tremmie pipe. Theoccurrence of shearing during the pumping process alters the slurryparticle size and can increase the slurry viscosity.

The inventive slurry composition has a set time greater than one hourallowing sufficient time for deployment. The slurry is pumped into theborehole 11 through a hose and the borehole 11 is preferably filled withslurry from the bottom up to prevent bridging and voids. In situ, thebackfill provides a thermal path from the outer casing 12 to theborehole wall. The grout backfill most preferably includes a sufficientamount of the thermal conductivity enhancing additive to provide athermal conductivity of greater than 3 Btu/hr-ft-F.

In a particularly preferred embodiment, the grout backfill used in thepresent invention is prepared for pumping into the borehole by (a)dissolving a thinning agent in water to form a dilute treated watersolution and then (b) adding a dry mix of any type discussed above tothe dilute treated water solution to form a pumpable grout backfillslurry. The dry mix preferably comprises, and more preferably consistsessentially of or consists solely of a pre-blended mixture of (a) fromabout 70% to about 85% by weight (more preferably about 70% to about 80%and most preferably about 75% by weight) natural flake graphite and (b)from about 30% to about 15% by weight (more preferably about 30% toabout 20% and most preferably about 25% by weight) sodium bentonite. Thenatural flake graphite preferably has a particle size of not greaterthan 200 mesh and the sodium bentonite preferably has a particle size ofnot greater than 325 mesh, more preferably in the range of from 200 to325 mesh.

The amount of thinning agent used in this embodiment for forming thedilute treated water solution is preferably an amount sufficient suchthat a pumpable slurry can be formed comprising, consisting essentiallyof, or more preferably consisting solely of (a) at least 30% by weight,more preferably from about 35% to about 45% by weight and mostpreferably about or at least 40% by weight of the dry mix and (b) notmore than 70% by weight, more preferably from about 65% to about 55% byweight, and most preferably about or not more than 60% by weight of thedilute treated water solution.

The inventive grout slurry described above consisting solely of 75 partsby weight (pbw) natural flake graphite, 25 pbw sodium bentonite andsufficient water and NaCl thinning agent to provide a slurry solidscontent of at least 30% by weight will have a superior thermalconductivity when hardened. Moreover, the thermal conductivity of thisinventive grout slurry will be further enhanced when blended to a slurrysolids content of at least 35% by weight and will be even greater whenblended to a slurry solids content of at least 40% by weight.

Examples of suitable thinning agents include, but are not limited to,sodium chloride, potassium chloride, and Aqua-Clear® PFD available fromBaroid. If the thinning agent used is sodium chloride and/or potassiumchloride, the dilute treated water solution will preferably comprise(and will more preferably consist essentially of and most preferablyconsist solely of) from about 8 to about 24 ounces by volume, morepreferably from about 12 to about 20 ounces, of sodium chloride and/orpotassium chloride per 9 gallons of water.

In the same way, Aqua-Clear® PFD will preferably be used in an amount offrom about 5 to about 15 milliliters, more preferably from about 8 toabout 10 milliliters, per 9 gallons of water. By way of example, theaddition of 9 milliliters of Aqua-Clear® PFD per each 9 gallons of waterprovides a PFD concentration in the treated water of only 0.026% byvolume.

In forming the preferred dry mix, the natural flake graphite and sodiumbentonite components can be pre-blended in the appropriate ratio using ahopper and then poured into, e.g., 50 lb. sacks. By way of example, ifno thinning agent is used, the addition of 12 gallons of untreated waterto each 50 lb. sack of dry mix will produce a pumpable slurry having asolids content of about 30% by weight. In contrast, by using a thinningagent as discussed above, a pumpable slurry having a solids content ofabout 40% by weight can be produced using only 9 gallons of treatedwater per 50 lb. sack of dry mix.

As will be apparent, all slurry component ratios stated herein and inthe claims relative to 50 lb. sack amounts of dry mix are equallyapplicable to slurry compositions containing less or more than 50 totalpounds of dry mix. Thus, for example, the addition of 0.9 gallons oftreated water to 5 lb. of dry mix is equivalent to and would be coveredby any reference herein to the addition of treated water in an amountequivalent to 9 gallons of treated water per each 50 lbs. of dry mix.

When a preferred grout backfill slurry having a 40% solids context ispumped into the borehole outside of the casing, the slurry will settlein the annulus to a solids content of about 50% solids content afterabout six hours, thus greatly increasing the physical point-to-pointcontact between the grout carbon particles. This, in turn, provides asignificantly enhanced level of thermal conductivity by allowing morephonons (infrared heat) to more easily move from one carbon particle tothe next without having to jump across large insulating gaps formed ofbentonite clay and bound water. Moreover, after about one month, thethinning agent will simply leach into the formation and the bentoniteclay will swell by absorption of bound water to reduce the waterpermeability of the inventive grout backfill to near zero.

The thinning agent used in the present invention is preferably sodiumchloride, potassium chloride, or a combination thereof, and is mostpreferably sodium chloride.

EXAMPLE 1

A grout slurry was formed by adding 246 g of treated water (consistingof 240 g water and 6 g sodium chloride) to a dry mix consisting of (a)120 g of natural flake graphite having a particle size passing (i.e.,less than) 200 mesh and (b) 40 g of sodium bentonite having a particlesize passing 325 mesh. The salt content of the treated water was thus2.44% by weight (equivalent to about 1.88 lb. NaCl per 9 gallons ofwater) and the solids content of the slurry product composition wasabout 40% by weight. The slurry product was pumpable and had a specificgravity of 1.25. After setting overnight, a layer of water had begun toform on top of the slurry composition by the following morning.

EXAMPLE 2

A grout slurry was formed by adding 152 g of treated water (consistingof 150 g water and 2 g sodium chloride) to a dry mix consisting of (a)75 g of natural flake graphite having a particle size passing a 200 meshscreen and (b) 25 g of sodium bentonite having a particle size alsopassing a 200 mesh screen. The salt content of the treated water wasthus 1.3% by weight (equivalent to 1 lb. of NaCl per 9 gallons of water)and the solids content of the slurry product composition was about 40%by weight. The slurry was more viscous, was not pumpable, was barelypourable, and had set up after one day. No water layer formed on the topof the composition.

EXAMPLE 3

A grout slurry was formed by adding 154 g of treated water (consistingof 150 g water and 4 g sodium chloride) to a dry mix consisting of (a)75 g of natural flake graphite having a particle size passing a 200 meshscreen and (b) 25 g of sodium bentonite having a particle size passing200 mesh. The salt content of the treated water was thus 2.6% by weight(equivalent to 2 lb. NaCl per 9 gallons of water) and the solids contentof the slurry product composition was about 40% by weight. The slurryproduct was pumpable. After one day, the slurry was thicker, but stillpourable, and 4 to 5 g of water had formed on the top of thecomposition.

These examples illustrate the effectiveness of the use of a thinningagent in accordance with the present invention for forming anddelivering the inventive grout backfill composition. In regard to theuse of sodium chloride, potassium chloride, or a mixture thereof as thethinning agent, these examples particularly show the effectiveness oftreated water compositions preferably consisting of from about 1.5% toabout 3% by weight, more preferably about 2.44% by weight of sodiumchloride and/or potassium chloride to form pumpable inventive groutslurry compositions having very high solids concentrations and excellentheat transfer characteristics.

Aqua-Clear® PFD, available from Baroid, is a liquid, phosphate-free,dispersant which comprises from 30 to 60% anionic polyacrylamide.Aqua-Clear® PFD has a pH of 6.5-7.5, a specific gravity of 1.2-1.4 at20° C., and is partially soluble in water. In contrast to the surprisingnew use thereof discovered in accordance with the present invention,Aqua-Clear® PFD has heretofore only been used, to our knowledge, at aconcentration of 0.2% by volume in water (i.e., almost an order ofmagnitude higher than the present invention) for (a) dispersing mud,sediment, and clay from producing formations and gravel packs and (b)reducing the viscosity and gel strength of drilling fluids.

An embodiment 202 of the improved supply and return header provided bythe present invention is illustrated in FIGS. 9, 10, and 12. Anembodiment 200 of an improved concentric ground exchange assembly havingthe inventive supply and return header 202 installed thereon isillustrated in FIG. 9.

The inventive supply and return header 202 comprises: an upper housing208 which preferably has a cylindrical “can” shape with exterior threads210 provided around the lower end portion thereof; a flat top plate 212which seals the upper end of the housing 208 and defines the upper endof the header assembly 202; a threaded inlet port 214 provided laterallythrough the vertical cylindrical wall 216 of the housing; a threadedoutlet port 218 which is also provided laterally through the verticalcylindrical housing wall 216, most preferably at a location in the rangeof from 90 to about 180 degrees from the inlet port 214; and an interiorfeed conduit 220. The interior feed conduit 220 extends horizontallyfrom the inlet port 214 toward the interior center of the housing 208 toan internal elbow 225. The internal elbow 225 directs the remainder ofthe interior feed conduit 220 vertically downward toward, and preferablythrough, the bottom end 221 of the cylindrical housing 208 forattachment to the inner conduit 222 of the concentric ground exchangeassembly 200.

Although other materials can be used, the header housing 208 and theinterior feed conduit 220 are preferably formed of brass or stainlesssteel and are most preferably formed of 304 stainless steel. To absorbthe tensile stress produced by the weight of the attached inner conduit222 of the exchange assembly, the upper horizontal end 224 of theinterior feed conduit 220 is preferably welded to the cylindrical wall216 of the housing 208 at the inlet port 214. Alternatively, or inaddition, the upper horizontal portion 224 of the interior feed conduit220 can be welded to the interior bottom surface of the housing topplate 212. The lower end of the interior feed conduit 220 is preferablyconnected to the upper end of the exchange assembly inner conduit 222using a coupling or other structure 230 having interior threads whichare received on corresponding threads 232 provided around the upper endportion of the exchange assembly inner conduit 222.

The inventive supply and return header 202 is preferably secured on theupper end of the exchange assembly casing 206 using a socket-to-threadadaptor 204. Although other materials can also be used, thesocket-to-thread adaptor 204 is preferably formed of fiberglass andincludes: (a) an upper interior threaded portion 234 which is receivedon the lower exterior threaded portion 210 of the housing 208 and (b) asmooth cylindrical lower interior portion 236 which corresponds to andis received on the upper end of the casing 206. The socket-to-threadadaptor 204 is preferably secured and hermetically sealed around theupper end portion of the casing 206 by chemically fusing the lowercylindrical interior portion 236 of the adaptor 204 directly to theexterior cylindrical wall of the casing 206 using an adhesive or otherbonding material which is compatible with both the composition of theadaptor 204 and the composition of the casing 206.

The socket-to-thread adaptor 204 is preferably formed by first mixingliquid epoxy resin, an aromatic amine hardener, and chopped glassstrands in an industrial mixer to form a “prepreg.” The prepreg mixtureis then press molded under heat and pressure to form the finished part.An example of a commercially available fitting which is well suited foruse in the present invention is a Fiber Glass Systems 29S threadedadaptor, preferably not having any hand lay-up overwrap applied on themolded body of the fitting.

The heat transfer working fluid feed line 238 and return line 240 aretypically formed of a plastic such as high density polyethylene andextend horizontally underground toward and away from the ground exchangeassembly header 202. Consequently, as mentioned above, the feed line 238and return line 240 are commonly referred to as “laterals.” Toaccommodate the different expansion and contraction characteristics ofthe plastic laterals 238, 240 and the preferred stainless steel headerhousing 208, the feed and return laterals 238 and 240 are preferablysecured to the header inlet and outlet ports 214 and 218 usingconnectors 242 and 244. The connectors 242 and 244 are preferably formedof stainless steel and each has an externally threaded first end 246which is threadedly received in the appropriate inlet or outlet port 214or 218 and a threaded second end 248 to which the feed lateral 238 orreturn lateral 240 is threadedly connected. Adhesives are alsopreferably used for sealing the connections between the connectors 242and 244 and the inlet and outlet ports 214 and 218 and between theconnectors 242 and 244 and the feed and return laterals 238 and 240.

As an alternative to the use of a socket-to-thread adaptor 204, it willbe understood that cylindrical can housing 208 can be sized and thelower portion extended as needed to allow the housing itself to beplaced on the upper end of the casing 206 and directly bonded thereto asillustrated in FIG. 14. In order to enhance the direct bond and sealbetween the housing 208 and the casing 206 in the embodiment of FIG. 14,a series of interior tiers 235 of decreasing diameter from the bottom upwill preferably be formed in the lower portion of the can housing 208.

An alternative embodiment 302 of the inventive supply and return headeris depicted in FIG. 15. The inventive header 302 is similar to theinventive header 202 except that the inventive header 302 is a unitarycast structure. Examples of materials suitable for forming the inventivecast header 302 include, but are not limited to, stainless steel or afiberglass composite. The inventive header 302 is preferably formed of304 stainless steel.

The inventive cast header 302 comprises: an outer housing 308 preferablyhaving a cylindrical can shape with a substantially flat upper end 312and a smooth cylindrical or tiered (or grooved) lower end 310 sized tobe received on, and chemically bonded directly to, the upper end of thecasing 206; an inlet port 314 projecting laterally from the verticalcylindrical wall 316 of the housing and optionally having threads cut inthe interior thereof for threadedly receiving the inlet lateral or aninlet compression fitting; an outlet port 318 projecting laterally fromthe housing wall 316 and optionally having threads cut in the interiorthereof for receiving the outlet lateral or an outlet compressionfitting; and an interior feed conduit 320.

The interior conduit 320 formed in the cast structure 302 comprises: (a)an upper portion which extends horizontally from the housing inlet 314and has an internal elbow 325 at the distal end thereof and (b) avertical discharge portion 327 which preferably extends from theinternal elbow 325 through the bottom end of the outer housing 308.Interior threads can also optionally be cut in the lower end portion 330of interior conduit 320 for either directly threadedly receiving theupper end of the inner conduit of the ground exchange system orthreadedly receiving an attachment fitting for the ground exchange innerconduit.

As with the inventive header 202, appropriate adhesives or other bondingagents will also preferably be used with the inventive cast header 202to further seal, bond, and secure all of the threaded direct or threadedfitting attachments between the header 302 and the feed and returnlaterals and the ground heat exchange inner conduit.

In addition, it will also be understood that: (a) external rather thaninternal threads could alternatively be cut into the cast header 302 foruse in some or all of the various threaded attachments, (b) rather thancutting threads in the cast header 302, some or all of the variousthreaded attachments could alternatively be replaced with smooth,grooved, or tiered bore direct adhesive attachments, and/or (c) ratherthan bonding the header housing 308 directly on the upper end of theground system casing, threads could alternatively be cut around theexterior of the cast header housing 308 for attachment to the upper endof the casing using a socket-to-thread adaptor in the same manner asdescribed above for inventive header 202.

In the inventive concentric ground exchange assembly 200, as illustratedin FIG. 14, the casing 206 can be any type of casing suitable for use inground exchange assemblies and will preferably comprise an stringassembly of fiber reinforced composite casing segments as describedabove. The composite casing segments will preferably be formed of athermoplastic material, most preferably epoxy, which is reinforced withfiberglass, carbon fiber, aramid fiber, or a combination thereof. Inaddition, the casing segments will preferably be formed by filamentwinding and will also preferably include at least 1.5% by weight of athermal conductivity enhancing additive such as aluminum flake, aluminumpowder, aluminum oxide, aluminum nitride, graphite, boron nitride,silicone carbide, Raney nickel, silver-coated nickel, silver-coatedcopper, or a combination thereof. An example of a commercially availableadhesive material which is well suited for bonding the preferredfiberglass socket-to-thread adaptor 204, the stainless steel housing208, or the cast stainless header 302 to a casing 206 of this type isWELDFAST® ZC-275 epoxy adhesive available from Fiber Glass Systems.

An example of a commercially available adhesive which is well-suited foruse in sealing the attachment between the materially different HDPEinlet and return laterals 238 and 240 and the stainless steel headerconnectors 242 and 244 is PLEXUS® MA830 methacrylate adhesive availablefrom ITW PLEXUS.

The inner conduit 222 employed in the inventive concentric ground heatexchange assembly 200 can be any type of inner pipe or other conduitused in concentric ground heat exchange assemblies but, as discussedabove, will preferably be a corrugated plastic or rubber drop tubehaving a continuous series of circular radial ribs 44, 250 provided onthe exterior thereof as illustrated in FIGS. 6 and 9. The corrugatedinner drop tube 222 will most preferably be formed of high densitypolyethylene, PVC, or EPDM (i.e., a terpolymer elastomer produced fromethylene-propylene diene monomer).

Because the 90° bend 235 or 325, which directs the horizontal flow fromthe feed lateral 238 into the vertical exchange assembly inner conduit222, is located within the housing 208 or 308 of the inventive supplyand return header 202 or 302, the height of the inventive supply andreturn header 102 can be as little as 5.25 inches or less, as comparedto a typical height of from 16 to 20 inches or more for the prior artheader 12 described above. This significantly reduces the amount oftrenching and digging required for the assembly and installation of theinventive concentric ground exchange assembly 200. In addition, theinventive supply and return header 202 or 302 provides a secure,hermetically sealed connection with the assembly casing 206 whicheliminates the need for flanges, flange rings, bolts, nuts, washers, andgaskets. Further, the stainless steel materials preferred for use informing the inventive supply and return header 202 or 302 are alsocommonly used with great success in other underground applications suchas water wells and do not present any regulatory difficulties orconcerns.

Thus, the present invention is well adapted to carry out the objectivesand attain the ends and advantages mentioned above as well as thoseinherent therein. While presently preferred embodiments have beendescribed for purposes of this disclosure, numerous changes,adaptations, and modifications will be apparent to those of ordinaryskill in the art. Such changes, adaptations, and modifications areencompassed within this invention as defined by the claims.

1. A grout slurry composition for conductive heat transfer applicationscomprising: a dry base premixed composition consisting of from about 70to about 85 parts by weight natural flake graphite and from about 30 toabout 15 parts by weight bentonite and water in an amount equivalent tonot more than 12 gallons of said water per each 50 pounds of said drybase premixed composition, said natural flake graphite having a particlesize of not greater than 200 mesh.
 2. The grout slurry composition ofclaim 1 wherein dry base premixed composition consists of said naturalflake graphite and sodium bentonite.
 3. The grout slurry composition ofclaim 2 wherein: said dry base premixed composition consists of fromabout 75 to about 80 parts by weight of said natural flake graphite andfrom about 25 to about 20 parts by weight of said sodium bentonite andsaid amount of said water is equivalent to an amount in a range of fromabout 8 to about 11 gallons of said water per each 50 pounds of said drybase premixed composition.
 4. The grout slurry composition of claim 1further comprising sand in an amount equivalent to not more than 25pounds of said sand per each 50 pounds of said dry base composition. 5.The grout slurry composition of claim 1 wherein said natural flakegraphite is present in said grout slurry premixed composition in anamount effective such that, when said grout slurry composition dries toform a set grout material, said set grout material will have a thermalconductivity of at least 3 Btu/hr-ft/° F.
 6. The grout slurrycomposition of claim 1 wherein said dry base premixed composition andsaid water are present in concentrations such that said grout slurrycomposition has a solids content of at least 35% by weight based on thetotal weight of said grout slurry composition.
 7. The grout slurrycomposition of claim 6 further comprising a thinning agent wherein: saidthinning agent is sodium chloride, potassium chloride or a combinationthereof said thinning agent is present in said grout slurry compositionin an amount effective such that said grout slurry composition is apumpable slurry.
 8. The grout slurry composition of claim 6 wherein saidthinning agent is present in said grout slurry composition in an amountequivalent to at least 8 ounces, by volume, of said thinning agent pereach 9 gallons of said water.
 9. The grout slurry composition of claim 8wherein said dry base premixed composition and said water are present insaid grout slurry composition in concentrations such that said groutslurry composition has a solids content of at least 40% by weight basedon the total weight of said grout slurry composition.
 10. The groutslurry composition of claim 1 further comprising a thinning agentcomprising at least one anionic polyacrylamide, wherein: said dry basepremixed composition and said water are present in said grout slurrycomposition in concentrations such that said grout slurry compositionhas a solids content of at least 35% by weight based on the total weightof said grout slurry composition and said thinning agent is present insaid grout slurry composition in an amount effective such that saidgrout slurry composition is a pumpable slurry.
 11. A grout slurrycomposition comprising: from about 70 to about 85 parts by weightnatural flake graphite; from about 30 to about 15 parts by weight ofbentonite; water in an amount effective such that said grout slurrycomposition has a solids content of at least 30% by weight based on thetotal weight of said grout slurry composition; and a thinning agent,wherein said thinning agent is sodium chloride, potassium chloride, or acombination thereof and said thinning agent is present in said groutslurry composition in an amount effective such that said grout slurrycomposition is a pumpable slurry.
 12. The grout slurry composition ofclaim 11 wherein said amount of said water is effective such that saidgrout slurry composition has a solids content of at least 35% by weightbased on the total weight of said grout slurry composition.
 13. Thegrout slurry composition of claim 12 wherein said amount of said wateris effective such that said grout slurry composition has a solidscontent of at least 40% by weight based on the total weight of saidgrout slurry composition.
 14. The grout slurry composition of claim 12wherein said natural flake graphite has a particle size of not greaterthan 200 mesh and said bentonite is sodium bentonite having a particlesize of not greater than 325 mesh.
 15. The grout slurry composition ofclaim 14 wherein said grout slurry composition consists solely of saidnatural flake graphite, said sodium bentonite, said water, and saidthinning agent.
 16. The grout slurry composition of claim 15 whereinsaid amount of said water is effective such that said grout slurrycomposition has a solids content of at least 40% by weight based on thetotal weight of said grout slurry composition.
 17. The grout slurrycomposition of claim 15 wherein said amount of said thinning agent isequivalent to at least 8 ounces by volume per each 9 gallons of saidwater.
 18. A method of installing a casing of a subterranean ground heatexchange system comprising the steps of: (a) placing said casing in aborehole; (b) forming treated water by adding a thinning agent to water;(c) adding said treated water to a dry base composition to form a groutslurry; and (d) placing said grout slurry in an annulus between aninterior wall of said borehole and an outer wall of said casing; whereinsaid dry base composition consists essentially of from about 70 to about85 parts by weight natural flake graphite and from about 30 to about 15parts by weight bentonite, said treated water is added to said dry basecomposition in an amount such that said grout slurry formed in step (c)has a solids content of at least 30% by weight based on the total weightof said grout slurry, and said thinning agent is added to said water instep (b) in an amount effective to allow said grout slurry to be placedin said annulus in accordance with step (d) by pumping.
 19. The methodof claim 18 wherein said thinning agent comprises at least one anionicpolyacrylamide.
 20. The method of claim 19 wherein said treated water isadded to said dry base composition in an amount such that said groutslurry formed in step (c) has a solids content of at least 35% by weightbased on the total weight of said grout slurry.
 21. The method of claim18 wherein said thinning agent is sodium chloride, potassium chloride,or a combination thereof.
 22. The method of claim 21 wherein saidtreated water is added to said dry base composition in an amount suchthat said grout slurry formed in step (c) has a solids content of atleast 35% by weight based on the total weight of said grout slurry. 23.The method of claim 22 wherein said dry base composition consists of:said natural flake graphite having a particle size of not greater than200 mesh and sodium bentonite having a particle size of not greater than325 mesh.
 24. The method of claim 23 wherein said thinning agent isadded to said water in step (b) in an amount equivalent to at least 8ounces by volume per each 9 gallons of said water.
 25. The method ofclaim 23 wherein said treated water is added to said dry basecomposition in an amount such that said grout slurry formed in step (c)has a solids content of at least 40% by weight based on the total weightof said grout slurry.
 26. The method of claim 23 wherein said groutslurry formed in step (c) and placed in said annulus by pumping in step(d) consists solely of: said dry base composition; said water, and saidthinning agent.
 27. In an apparatus for subterranean ground heatexchange comprising a casing extending underground, an internal deliveryconduit extending into said casing, and a supply and return headermounted at an upper end of said casing for supplying a heat transferfluid into said internal delivery conduit and for receiving said heattransfer fluid as it returns to said supply and return header via areturn flow annulus between said internal delivery conduit and aninterior wall of said casing, the improvement comprising said supply andreturn header comprising: a housing having a closed top and a verticallyextending outer wall which surrounds an interior of said housing, saidclosed top defining an upper end of said supply and return header; asupply port for said heat transfer fluid provided through saidvertically extending outer wall; a return port for said heat transferfluid provided through said vertically extending outer wall; and aninterior supply conduit including a first portion of said interiorsupply conduit which extends into said interior of said housing fromsaid supply port and has a bend which is positioned in said interior ofsaid housing for directing said heat transfer fluid downwardly and asecond portion of said interior supply conduit which extends downwardlyfrom said bend for delivering said heat transfer fluid to said internaldelivery conduit.
 28. The apparatus of claim 27 wherein the improvementfurther comprises said second portion of said interior supply conduitextending downwardly through a lower end of said housing.
 29. Theapparatus of claim 27 wherein the improvement further comprises a collarfor mounting said housing on said upper end of said casing, said collarhaving an upper portion wherein a lower end of said housing is receivedand said collar having a lower portion wherein said upper end of saidcasing is received.
 30. The apparatus of claim 29 wherein theimprovement further comprises said collar being a socket-to-threadcollar comprising interior threads in said upper portion of said collarfor threaded attachment to corresponding exterior threads providedaround said outer wall of said housing and said lower portion of saidcollar being a socket wherein said upper end of said casing is receivedfor bonding said collar to an exterior upper end portion of said casing.31. The apparatus of claim 27 wherein the improvement further comprisessaid housing, said supply port, said return port, and said interiorsupply conduit being integrally formed as a unitary cast structure. 32.The apparatus of claim 27 wherein the improvement further comprises saidclosed top having a substantially flat upper exterior surface.
 33. Theapparatus of claim 27 wherein the improvement further comprises saidsupply and return header having a height effective such that, as mountedon said upper end of said casing, said upper end of said supply andreturn header is not more than 10 inches above said upper end of saidcasing.
 34. The apparatus of claim 27 wherein the improvement furthercomprises said supply and return header having a height effective suchthat, as mounted on said upper end of said casing, said upper end ofsaid supply and return header is not more than 8 inches above said upperend of said casing.
 35. The apparatus of claim 27 wherein theimprovement further comprises said supply and return header having aheight effective such that, as mounted on said upper end of said casing,said upper end of said supply and return header is not more than 7inches above said upper end of said casing.
 36. The apparatus of claim27 wherein the improvement further comprises said supply and returnheader having a height effective such that, as mounted on said upper endof said casing, said upper end of said supply and return header is notmore than 6 inches above said upper end of said casing.
 37. Theapparatus of claim 27 wherein the improvement further comprises saidhousing having a lower interior portion which is sized and adapted suchthat said upper end of said casing is received in said lower interiorportion of said housing and said lower interior portion of said housingis directly bonded to an exterior upper end portion of said casing. 38.The apparatus of claim 37 wherein the improvement further comprises saidlower interior portion of said housing comprising a plurality ofinterior tiers which are of decreasing diameters from a lower end ofsaid housing upward.
 39. The apparatus of claim 27 wherein theimprovement further comprises a series of discrete, space apart, radialribs extending along an exterior of said internal delivery conduit suchthat said radial ribs project into said return flow annulus.
 40. Theapparatus of claim 39 wherein the improvement further comprises saidseries of discrete, spaced apart, radial ribs having a ratio ofpeak-to-peak rib spacing to radial peak height in the range of fromabout 1.2:1 to about 3:1.
 41. The apparatus of claim 39 wherein theimprovement further comprises said internal delivery conduit beingcorrugated tubing.
 42. The apparatus of claim 41 wherein the improvementfurther comprises said radial ribs being circular.
 43. The apparatus ofclaim 27 wherein the improvement further comprises said casingcomprising a string of filament wound pipe segments formed from athermosetting plastic composition and a reinforcing fiber materialwherein: said reinforcing fiber material is fiberglass, carbon filter,aramid fiber or a combination thereof; said thermosetting plasticcomposition includes a thermal conductivity enhancing additive; and saidthermal conductivity enhancing additive is medium grade aluminum powderpresent in said thermosetting plastic composition in an amount of atleast 1.5% by weight based on total weight of said thermosetting plasticcomposition; and said filament wound pipe segments have an outsidediameter to inside diameter ratio of not more than 1.055 and a collapsepressure of at least 75 psig.